Dopamine receptors are reported to undergo gradual and selective changes with aging, both in rodents and in humans when measured in vitro, but the functional significance of these changes is not well understood. In order to investigate this phenomenon, a pilot study will be initiated which will examine the behavioral and neurochemical effects of systemic administration of selected dopaminergic agonists in mice of different ages (2, 6, 18 and 24 mos.). Mice will receive the standard drug, apomorphine, and two selective drugs, SKF38393 (for D1 receptor activity) and LY141865 (for D2 activity) and their effect on motor activity and on dopamine utilization (using dopamine metabolite formation as an index) will be measured in the different age groups. It is anticipated that age-related changes in the responsiveness to these drugs will be observed, if the changes in dopamine receptors reflects significant changes in brain function. These studies should contribute to a better understanding of the rationale for the use of these drugs in age-related neurological and behavioral disorders.